The present disclosure relates to a storage element and a storage apparatus that include a plurality of magnetic layers and record data using spin torque magnetization reversal.
The present disclosure also relates to a magnetic head that detects a magnetic signal from a magnetic recording medium.
The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2012-217702 filed in the Japan Patent Office on Sep. 28, 2012, the entire content of which is hereby incorporated by reference.
With the rapid development of various information appliances from mobile terminals to large-scale servers, even higher performance, such as higher integration, higher speed, and reduced power consumption, has been sought for components like memory and logic elements used to configure such appliances.
In particular, significant advances have been made in semiconductor nonvolatile memory, with flash memory as a large-capacity file memory becoming increasingly widespread and taking the place of hard disk drives.
Meanwhile, the development of nonvolatile semiconductor memories is also advancing in order to replace NOR flash memory, DRAM, and the like that are presently typically used for code storage and for working memory. FeRAM (Ferroelectric Random Access Memory), MRAM (Magnetic Random Access Memory), and PCRAM (Phase Change RAM) can be given as examples of such nonvolatile semiconductor memories. Some of these have already been commercialized.
Out of such nonvolatile memories, MRAM stores data using the direction of magnetization of a magnetic body and is therefore capable of high-speed rewriting and almost infinite rewrites (1015 times or more). MRAM is already in use in fields such as industrial automation and aircraft.
Due to its high-speed operation and reliability, there are high expectations on future use of MRAM as code storage and working memory.
However, reducing power consumption and increasing capacity remain as issues for MRAM. These are genuine problems due to the recording principles of MRAM, that is, an arrangement where magnetism is reversed using a current magnetic field generated from a wire.
As a method of solving the above problem, recording methods (that is, magnetic reversal) that do not rely on a current magnetic field are being investigated. Research relating to spin torque magnetization reversal is particularly active (see, for example, PTL 1, PTL 2, PTL 3, NPL 1, and NPL 2).
In the same way as MRAM, in many cases storage elements that use spin torque magnetization reversal are configured using MTJ (Magnetic Tunnel Junctions).
Such configuration uses the torque (also referred to as “spin transfer torque”) applied to a free magnetic layer (whose direction of magnetization is not pinned) when spin-polarized electrons, which have passed a magnetic layer that is pinned in a certain direction, enter the free magnetic layer, with the free magnetic layer reversing when a current of a given threshold or larger flows. Rewriting of 0/1 is carried out by changing the polarity of the current.
The absolute magnitude of a current for such reversal is 1 mA or below for an element of a scale of around 0.1 micrometers. Since such current magnitude decreases in proportion to the element volume, scaling is also possible. In addition, since word lines for generating a recording current magnetic field that were necessary with MRAM are unnecessary, there is a further advantage that the cell construction becomes simplified.
Hereinafter, a MRAM that uses spin torque magnetization reversal is referred to as a STT-MRAM (Spin Torque Transfer-based Magnetic Random Access Memory). Note that spin torque magnetization reversal is sometimes also referred to as “spin injection magnetization reversal”.
There are great expectations for STT-MRAM as a nonvolatile memory capable of reduced power consumption and increased capacity while maintaining the advantages of MRAM, i.e., high speed and the ability to perform almost infinite rewrites.
When an MTJ construction is applied to the construction of a storage element as an STT-MRAM, as one example a base layer, pinned magnetization layer, intermediate layer (insulating layer), storage layer, cap layer construction is used.
The merit of applying an MTJ construction is that a large rate of change in magnetoresistance can be ensured, which increases the read signal.
Here, since STT-MRAM is nonvolatile memory, it is necessary to stably store information written by a current. That is, it is necessary to ensure stability with respect to thermal fluctuations in magnetization of the storage layer (also referred to as “thermal stability”).
If thermal stability of the storage layer is not ensured, there can be cases where the reversed direction of magnetization is re-reversed due to heat (i.e., the temperature in the operating environment), resulting in write errors.
As described above, compared to an existing MRAM, an STT-MRAM storage element is advantageous for scaling, or in other words, has an advantage in that it is possible to reduce the volume of the storage layer. However, when the volume is reduced, if other characteristics remain the same, there is a tendency for a drop in thermal stability.
Since the volume of a storage element becomes significantly smaller if the capacity of STT-MRAM increases, ensuring thermal stability is an important issue.
For this reason, thermal stability is an extremely important characteristic for a storage element in an STT-MRAM and it is necessary to use a design where thermal stability is ensured even when the volume is reduced.
Here, it is important to note that the current flowing in a storage element is limited to the magnitude of a current (that is, the saturation current of a selection transistor) capable of flowing in a selection transistor (i.e., a transistor for selecting a storage element in which a current is to flow out of the storage elements that construct each memory cell). In other words, it is necessary to carry out a write into a storage element using a current at or below the saturation current of a selection transistor.
Since it is known that the saturation current of a transistor falls as the transistor is miniaturized, to enable miniaturization of STT-MRAM, there is demand to improve the efficiency of spin transfer so as to reduce the current supplied to a storage element.
Also, if a tunnel insulating layer is used in an intermediate layer as an MTJ structure, to prevent dielectric breakdown of the tunnel insulating layer, there is a limit on the magnitude of current supplied to a storage element. In other words, from the viewpoint of maintaining reliability for repeated writes of a storage element also, it is necessary to suppress the current that is necessary for spin torque magnetization reversal.
In this way, in an STT-MRAM storage element, there is demand to reduce the reversal current necessary for spin torque magnetization reversal to the saturation current of a transistor and a current at which breakdown occurs for an insulation layer (intermediate layer) as a tunnel barrier, or lower.
That is, for an STT-MRAM storage element, there is demand to ensure thermal stability as described earlier and to also reduce the reversal current.
To achieve both a reduction in the reversal current and maintain thermal stability, attention has been focused on a construction that uses a perpendicular magnetization film as a storage layer.
It has been suggested, according to NPL3, for example, that using a perpendicular magnetization film such as a Co/Ni multilayer film in the storage layer makes it possible both to reduce the reversal current and to ensure thermal stability.
There are a number of types of magnetic material with perpendicular magnetic anisotropy, such as rare earth-transition metal alloys (such as TbCoFe), metal multilayer films (such as a Co/Pd multilayer film), ordered alloys (such as FePt), and use of interface anisotropy between an oxide and a magnetic metal (such as Co/MgO). However, in view of the use of a tunnel junction construction to realize a high rate of change in magnetoresistance that provides a large read signal in an STT-MRAM and also in view of heat resistance and ease of manufacturing, a material that uses interface anisotropy is desirable, that is, a construction where a magnetic material including Co or Fe is laminated on Mg as a tunnel barrier.
Meanwhile, it is also desirable to use a perpendicular magnetization magnetic material that has interface magnetic anisotropy in a pinned magnetization layer. In particular, to provide a large read signal, it is desirable to laminate a magnetic material including Co or Fe immediately below MgO as a tunnel barrier.
To ensure thermal stability, it is effective to use a so-called “laminated ferri-pinned construction” as the construction of the pinned magnetization layer. That is, the pinned magnetization layer is a laminated construction of at least three layers made up of at least two ferromagnetic layers and a non-magnetic layer. Normally, a laminated ferri-pinned construction will often use a laminated construction composed of two ferromagnetic layers and a non-magnetic layer (Ru).
By using a laminated ferri-pinned construction as the pinned magnetization layer, it is possible to reduce the bias on the storage layer due to a magnetic field which leaks from the pinned magnetization layer and thereby improve thermal stability.